What Are Race Predictors All About?
The Question Everybody Asks
The question almost everyone wants answered is: What could I run for a marathon based on my...[enter another race in here] time?
It's a great question. It's also the quickest way I know to talk yourself into a goal you can't hit.
So here's what race predictors actually are, where they're genuinely useful, and where they'll quietly set you up for a miserable morning on race day.
One Race Result Unlocks the Rest
The idea behind every race predictor you've ever used is simple. As the distance goes up, your pace comes down, and it does it in a fairly predictable way.
You can't hold your 5k pace for a half marathon. You can't hold your half marathon pace for a full one. Every runner knows that instinctively. What's less obvious is that the slowdown is consistent enough across enough people that you can put a number on it.
A man called Peter Riegel did exactly that in 1977, in Runner's World. His formula takes a finishing time you already have, compares the two distances, and applies what's known as a fatigue factor. That's just a number representing how much you slow down as the race gets longer. Put in one result, get an estimate for all the others.
It's elegant, it's nearly fifty years old, and it's still the engine behind almost every free calculator on the internet.
Why the Pace Drops
Three things stack up against you as the miles go on.
The effort you can hold comes down. In a 5k you can run above the level you could sustain all day, because the race is over before that catches up with you. Over a marathon you have to stay well underneath it for hours. An effort that feels fine for twenty five minutes is a different proposition at three hours.
You run out of fuel. Your body stores roughly ninety minutes' worth of its preferred fuel, which is glycogen, the carbohydrate held in your muscles and liver. After that you're leaning more and more on fat, which works but is slower to convert. This is why fuelling matters so much in a marathon and barely registers in a 10k.
The damage builds. Every stride puts force through your legs. Over five kilometres that's manageable. Over forty two, it accumulates, your muscles get worse at doing their job, your form goes, and holding any given pace costs you more than it did an hour earlier.
Put those three together and you get a curve rather than a straight line. The further you go, the steeper it gets.
Where Most Calculators Let You Down
Riegel's original formula used one fatigue factor for everything: 1.06.
For shorter distances that's excellent. If you know your 5k time, it'll give you a solid estimate of your 10k. The gap between those two races just isn't big enough for much to go wrong.
The trouble starts when you ask it about a marathon.
The marathon isn't simply a longer race. It's a different event. Running out of fuel is a real possibility in a way it never is over 10k. Three or four hours on your feet makes demands that twenty five minutes doesn't. A single fatigue factor can't cope with that, and 1.06 in particular is far too generous.
So you put your parkrun PB into a free calculator, and it hands you a marathon time that looks fantastic.
You write it down. You tell people. And then at twenty miles you find out what the number was really worth.
What We Did About It
When we built the Race Predictor in our app, this was the thing we most wanted to fix. Rather than one fatigue factor for every question, it uses three, depending on how big a jump you're asking it to make.
Between the shorter distances, say 5k to 10k, it uses Riegel's 1.06. He was right about this bit.
Half marathon to marathon, it uses 1.085. A steeper curve, because the second half of a marathon asks far more of you than simply doing the half twice.
A short distance to the marathon, say 5k or 10k to 26.2 miles, it uses 1.10. That's the most cautious of the three, because it's the biggest leap and the one where the optimism does the most damage.
Put in a twenty five minute 5k and the 10k prediction comes out on the gentle factor, because that's the honest answer for that jump. The marathon prediction comes out on the steep one.
The results are less flattering. That's rather the point. I'd far rather hand you a number you can hit off the back of a good winter than one that reads beautifully in September and falls apart in April.
What a Prediction Actually Tells You
This is the bit worth being clear about, because it's where most of the misunderstanding lives.
A prediction shows your potential. It does not show your guarantee.
What it's really saying is this: the engine underneath that parkrun time could produce this marathon time, if you go and train specifically for a marathon. Months of long runs, practising your fuelling, getting used to being on your feet for hours. The potential is there. The training is what turns it into a result.
A twenty five minute parkrun does not mean you could go out next Sunday and run the marathon time next to it. Nobody can. That's not a failing of the maths, it's just what the maths is for.
One more thing on this. Predictions are built on race data, not training data, and that distinction matters. A 5k PB from a parkrun where you were fresh and racing hard is real evidence. A fast 5k in the middle of a long run is not, because you weren't racing. The only day your pace really matters is race day, and that's exactly the day the predictor wants to hear about.
A Compass, Not a GPS
I'll be honest with you: predicting race times is a bit of a fool's errand. Every race is different, every runner is different, and the weather, the course, your training, your sleep and your morning all get a say.
Some runners are naturally quick over short distances and their 5k makes their marathon look better than it is. Others barely trouble anyone over 5k and then come alive at twenty miles. The formula assumes you've prepared equally for every distance, which almost nobody has.
So use it, but use it like this:
Treat the numbers as estimates. Some people will go quicker. Some will go slower. They aren't promises.
Train for the distance you're racing. A marathon prediction from your 10k only becomes real with marathon specific work behind it.
Remember it knows nothing about the day. It hasn't seen the hills, the heat, or the headwind on the Embankment.
Use the most recent race you've got. A 5k from three years ago is a worse input than a half marathon from last month.
Have an A, B and C goal. A is the prediction. B is a few minutes slower. C is to finish strong and enjoy it. When conditions turn, having somewhere sensible to go keeps your head in the race instead of the whole thing collapsing at fourteen miles.
Think of it as a compass. It points you in roughly the right direction and tells you what's realistic. It is not a GPS. It cannot see every hill or every gust of wind, or the moment your body decides it's had enough.
So, Should You Use One?
Yes. Genuinely, yes.
Put a recent race in. Look at what comes out. Sit with it for a minute.
If the marathon number looks intimidating, good. It should. It's an enormous thing to do and respecting the distance is the first step to doing it well. If everything looks achievable, better still. Your fitness is in a decent place and you've got something to aim at.
Either way, take the number, talk it through with your coach if you've got one, and build the plan around it. The predictor tells you what's possible. The winter tells you whether it happens.
Working towards a spring marathon? This is exactly the point in the year where a goal time is worth getting right, before it's shaped six months of training. Drop us a line and we'll build a block around your life and the race you're actually capable of. Book a coaching package →
